In conventional vehicle engines, there is provided an intake manifold for introduction of intake air, which intake manifold has a mounting flange mounted on a cylinder head, a plurality of branch pipes successively arranged along the mounting flange, and a surge tank to which the branch pipes are connected.
More specifically, as illustrated in FIG. 5, an engine 102 has intake ports 106 formed in a cylinder head 104. An intake manifold 108 has branch pipes 110 which respectively communicate with the intake ports 106. In addition, the intake manifold 108 has a mounting flange 112 mountable on the cylinder head 104 at a side portion thereof, which mounting flange 112 is a metal, plate-like member. In this case, since the mounting flange 112 is formed of a metal plate, a fuel injection valve-mounting portion 114 is provided on the cylinder head 104. A fuel injection valve 116 is mounted on the mounting portion 114.
In general, the intake manifold is entirely formed either by a light alloy casting, such as an aluminum alloy, or by resin. When fabricated as a light alloy casting, a greater degree of freedom is present in shaping the intake manifold. However, an air passage (an air-fuel mixture passage) inside each of the branch pipes includes a coarser or rougher inner surface. This results in increased air flow resistance.
When formed by resin, then the intake manifold is lighter in weight and a greater degree of freedom is present in shaping the manifold. Further, the air passage has a smoother inner surface formed inside the branch pipe, and thus exhibits less ventilation or flow resistance. However, there is another inconvenience in that the strength/rigidity of the intake manifold is difficult to insure.
Further, there is also a known type of intake manifold formed by a combination of several individual members, in which the branch pipes are formed by iron series-containing pipe members, while the mounting flange is formed by an iron series-containing plate member (made of the plate metal).
When the branch pipes and the mounting flange are formed as individual members, the air passage inside each of the branch pipes has a smoother inner surface and thus ventilation resistance is reduced. Further, the intake manifold includes fewer worked portions. In addition, the intake manifold is less expensive. However, the intake manifold in this case has an inconvenience of having a lesser degree of freedom in the shaping thereof.
When the branch pipes and the mounting flange are formed by individual aluminum series-containing pipe members and an aluminum series-containing forged member, then the air passage inside each of the branch pipes has a smoother inner surface, and thus exhibits reduced ventilation resistance. However, the intake manifold in this case has yet another inconvenience of increased cost.
Examples of the above-discussed types of intake manifolds are disclosed in, e.g., published Japanese Examined Patent Application Nos. 7-45853 (1995) and 6-5061 (1994), and in published Japanese Patent No. 2539226 (1996). The intake manifold according to Japanese Patent Application No. 7-45853 includes a main pipe and a plurality of branch pipes. More specifically, the main pipe is open at one end, but is closed at the other end. In addition, the main pipe has a plurality of holes formed in a peripheral wall thereof. Further, an outwardly protruding flange is integrally provided surrounding each of the holes, through which flanges the branch pipes are connected to the main pipe. Each of the branch pipes is formed by extrusion. The extruded pipe is connected at one end to the above-mentioned outwardly protruding flange such that the two are snugly engaged with one another. Such a construction provides the intake manifold with a smooth inner surface, less air resistance, and improved intake efficiency.
Japanese Patent Application No. 6-5061 describes a method for producing the entire intake manifold in a simple manner, including the steps of: molding a plurality of aluminum-extruded pipe members, each of which includes a large number of linear grooves on an inner peripheral surface thereof, the linear grooves extending in the lengthwise direction of the pipe member; distorting each non-worked pipe member, thereby forming a spiral-like groove in the inner peripheral surface over the entire length thereof; bending the non-worked pipe members, thereby producing a plurality of branch pipes, each of which has the spiral-like groove defined in the inner peripheral surface over the entire length thereof; and, brazing the branch pipes onto a main pipe.
Lastly, according to Japanese Patent No. 2539226, a thicker portion is provided on a peripheral wall of a main pipe at a location where branch pipes are to be connected thereto. Holes are punched at the thicker portion, through which holes the branch pipes are connected to the main pipe. A radially extending portion is opened and formed at a peripheral surface of each of the holes. The radially extending portion extends from the outermost end by a predetermined length. One end of the branch pipe is inserted into the radially extending portion, and is then brazed to the main pipe. This process eliminates a need to provide both holes and cylindrically shaped, outwardly protruding portions for use in connection of branch pipes, as practiced in conventional manners. As a result, lower manufacturing costs and fewer manufacturing processes are realized.
In conventional intake manifolds, since the mounting flange is fabricated using a metal plate member, then the fuel injection valve-mounting portion cannot be provided on the mounting flange. Thus, the fuel injection valve must be mounted directly on the cylinder head. This brings about an inconvenience in that the fuel injection valve cannot be used on engines that do not have the fuel injection valve-mounting portion on the cylinder head.
With the mounting flange formed using a plate member made of metal plate, the fuel injection valve-mounting portion can be provided on the mounting flange by the same mounting flange being made larger in thickness. However, this causes inconveniences in that it takes a long time to work or form such thicker mounting flange, thereby increasing costs, and further potions of the mounting flange are also unnecessarily made larger in thickness, with a concomitant increase in weight.
Further, since the heat conductivity of an aluminum alloy and that of iron are 240 W.multidot.m.sup.-1 K.sup.-1 for 100.degree. C. and 72 W.multidot.m.sup.-1 K.sup.-1 for 100.degree. C., respectively, then the heat conductivity of the aluminum alloy is about three times as large as that of iron. Accordingly, with the intake manifold formed using the aluminum alloy, heat is readily conducted from the cylinder head to the intake manifold. The heat is then transferred to the fuel delivery pipes which include fuel passages, which pipes are provided on the branch pipes. As a result, fuel in the delivery pipes is heated. This causes another inconvenience in that harmful percolation (fuel volatility) is readily produced. A further inconvenience is that a complicated structure and high costs are involved when a fuel supply system is provided with a counter-measure to obviate the former inconvenience.